1. Field of the Invention
The present invention relates to a method of producing a polyester powdery coating material comprising a curable polyester resin.
2. Description of the Related Art
Powdery coating materials do not generate volatile organic compounds during the baking, do not cause environmental problems such as air pollution, and have, hence, been used in a wide field of applications to substitute for the solution coating materials.
Such powdery coating materials are obtained by blending a curable resin with a curing agent, and can be represented by acrylic powdery coating materials using, as a curable resin, a curable acrylic resin having a glycidyl group or a hydroxyl group, polyester powdery coating materials using a curable polyester resin having a hydroxyl group or a carboxyl group, and epoxy powdery coating materials using an epoxy resin. As the curing agents for the acrylic powdery coating materials, there have been known an aliphatic dibasic acid, a blocked isocyanate, an uretdione and an amino compound. As the curing agents for the polyester powdery coating materials, there have been known a blocked isocyanate, an uretdione, an amino compound, an epoxy compound and a glycidyl compound. As the curing agents for the epoxy powdery coating materials, there have been known a dicyanediamide and an acid anhydride.
The powdery coating materials have heretofore been produced by dry-mixing pigments and other additives for the coating materials in addition to the curable resin and curing agent, kneading and dispersing them by using a melt-kneader, pulverizing them and classifying them.
In heating and melt-kneading the curable resin and the curing agent according to this method, however, it is necessary to suppress the crosslinking reaction between the curable resin and the curing agent. Therefore, limitation is imposed such as conducting the kneading within a short period of time and effecting the kneading at a temperature lower than the melting point or the softening point of the curing agent, making it difficult to homogeneously disperse the curing agent. Namely, with the powdery coating material in which the curing agent is not homogeneously dispersed, the film formed therefrom lacks appearance and, particularly, smoothness. Besides, the pigment is poorly dispersed, and the powdery coating material makes it difficult to obtain a film that exhibits vividness, sharpness and gloss comparable to those of liquid coating materials.
In order to improve such problems, therefore, there have been proposed methods of mixing the starting materials of the powdery coating material, such as a curable resin and a curing agent in a wet manner, i.e., in a solvent [Japanese Unexamined Patent Publication (Kokai) No. 25531/1979 (a), Japanese Unexamined Patent Publication (Kokai) No. 53729/1998 (b), Japanese Unexamined Patent Publication (Kokai) No. 302567/1999 (c), Japanese Unexamined Patent Publication (Kokai) No. 349859/1999 (d), Japanese Unexamined Patent Publication (Kokai) No. 034426/2000 (e) and Japanese Unexamined Patent Publication (Kokai) No. 103866/2000 (f)]. There have been disclosed that these methods make it possible to highly homogeneously disperse the curable resin and the curing agent.
However, the methods proposed by the above related arts (a), (d) and (e) are adapted to the acrylic powdery coating materials but cannot be adapted to the polyester powdery coating materials. That is, the polyester powdery coating materials use a resin and a curing agent different from those of the acrylic powdery coating materials and, hence, the solvents used by these related art cannot be readily used. Another reason is that the polyester powdery coating materials have properties that are basically different from the properties of the acrylic powdery coating materials in regard to weather resistance or chip resistance.
Further, though the prior arts (b) and (c) are not particularly specifying the kinds of the powdery coating materials, what are concretely disclosed therein are acrylic powdery coating materials, and there is no concrete disclosure concerning the polyester powdery coating materials. In particular, the related art (b) discloses the use of acrylic resin solutions and curing agent solutions. These solutions, however, use different solvents. When these solvents are mixed together, therefore, the curing agent is often precipitated. Besides, the solutions are very lowly soluble in one another leaving a problem concerning the homogeneous dispersion of the curing agent. The related art (c) discloses an example of using a solvent which does dissolve the curing agent at all. In this case, too, a problem remains concerning the homogeneous dispersion of the curing agent.
The related art (f) is concerned with a method of producing an acrylic powdery coating material and a polyester powdery coating material by spray-drying, using a solvent which dissolves both the curable resin and the curing agent. However, the curing agent or the pigment easily breeds out on the surface of the powdery coating material obtained by spray-drying, leaving a problem in regard to homogeneity in the crosslinked density and the additive, and a film having a high degree of finish, appearance and properties is not still obtained.
Besides, the methods disclosed in the above-mentioned related arts (a) to (f) still leave many problems such as the necessity of vessels for dissolving starting coating materials, mixing devices for mixing the solutions thereof, and a large vapor-removing/recovering apparatus for removing large amounts of the solvent.